Support for basic if/elif directives

[belav]

closes #15

This handles the basics of formatting code in if directives and supplies a configuration option for more complex cases.
#404 will improve upon this initial work

[belav]

I tested this across the whole of https://github.com/belav/csharpier-repos and came across

• An unrelated issue with switch/pattern matching that just happened to be in an #if SwitchExpression + Pattern edge cause causes extra line and poor formatting #408
• An edge case that feels low priority When #if starts with comma, it formats oddly #410
• Instances of files that won't compile because they have bad code inside of an #if

There may be some other edge cases similar to #410, I haven't reviewed the actual formatting changes this makes except on the aspnetcore repo. But I think this can be deemed good enough for the first pass.